Study GuideBiology–Animals: Invertebrates1. CnidariaCnidariais a group of animals that includeshydras, jellyfish, sea corals, and sea anemones. Mostcnidarians live inmarine (ocean) environments, although a few can be found infreshwater.1.1 Body Structure and SymmetryCnidarians have atissue-level organization, which means their bodies are made of tissues but notcomplex organs. Their body plan showsradial symmetry. This means their bodies are arranged in acircular way, with body parts spreading outward from the center—like the spokes of a wheel.1.2 Tentacles and Stinging CellsAround the body opening, cnidarians havetentacles. These tentacles contain special stinging cellscalledcnidocytes. Cnidocytes help the animal:•Defend itselffrom predators•Capture food, such as small animals in the waterWhen triggered, these stinging cells release toxins that immobilize prey.1.3 Digestion and the Gastrovascular CavityCnidarians digest food inside a central space called thegastrovascular cavity. This cavity has twomain roles:•It helps withdigestion•It also helpsdistribute nutrientsthroughout the bodyCells lining the cavity releasedigestive enzymesthat break food down. The digested nutrients arethen absorbed directly into the cells.Preview Mode
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